NAME

     setuid, seteuid, setgid, setegid — set user and group ID

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     int
     setuid(uid_t uid);

     int
     seteuid(uid_t euid);

     int
     setgid(gid_t gid);

     int
     setegid(gid_t egid);

DESCRIPTION

     The setuid() system call sets the real and effective user IDs and the saved set-user-ID of
     the current process to the specified value.  The setuid() system call is permitted if the
     specified ID is equal to the real user ID or the effective user ID of the process, or if the
     effective user ID is that of the super user.

     The setgid() system call sets the real and effective group IDs and the saved set-group-ID of
     the current process to the specified value.  The setgid() system call is permitted if the
     specified ID is equal to the real group ID or the effective group ID of the process, or if
     the effective user ID is that of the super user.

     The seteuid() system call (setegid()) sets the effective user ID (group ID) of the current
     process.  The effective user ID may be set to the value of the real user ID or the saved
     set-user-ID (see intro(2) and execve(2)); in this way, the effective user ID of a set-user-
     ID executable may be toggled by switching to the real user ID, then re-enabled by reverting
     to the set-user-ID value.  Similarly, the effective group ID may be set to the value of the
     real group ID or the saved set-group-ID.

RETURN VALUES

     Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and
     the global variable errno is set to indicate the error.

ERRORS

     The system calls will fail if:

     [EPERM]            The user is not the super user and the ID specified is not the real,
                        effective ID, or saved ID.

SEE ALSO

     getgid(2), getuid(2), issetugid(2), setregid(2), setreuid(2)

STANDARDS

     The setuid() and setgid() system calls are compliant with the ISO/IEC 9945-1:1990
     (“POSIX.1”) specification with _POSIX_SAVED_IDS not defined with the permitted extensions
     from Appendix B.4.2.2.  The seteuid() and setegid() system calls are extensions based on the
     POSIX concept of _POSIX_SAVED_IDS, and have been proposed for a future revision of the
     standard.

HISTORY

     The setuid() function appeared in Version 1 AT&T UNIX.  The setgid() function appeared in
     Version 4 AT&T UNIX.

SECURITY CONSIDERATIONS

     Read and write permissions to files are determined upon a call to open(2).  Once a file
     descriptor is open, dropping privilege does not affect the process's read/write permissions,
     even if the user ID specified has no read or write permissions to the file.  These files
     normally remain open in any new process executed, resulting in a user being able to read or
     modify potentially sensitive data.

     To prevent these files from remaining open after an exec(3) call, be sure to set the close-
     on-exec flag:

     void
     pseudocode(void)
     {
             int fd;
             /* ... */

             fd = open("/path/to/sensitive/data", O_RDWR | O_CLOEXEC);
             if (fd == -1)
                     err(1, "open");

             /* ... */
             execve(path, argv, environ);
     }